Preconception thyroid-stimulating hormone and pregnancy outcomes in women with hypothyroidism

Maternal hypothyroidism may adversely affect pregnancy outcomes. International practice guidelines recommend that women with hypothyroidism should attain a preconception and early gestation serum thyroid-stimulating hormone (TSH) level of <2.5 mU/L. Our objective was to ascertain what proportion of women realize this target in practice and whether a TSH level above this threshold has adverse fetal and maternal consequences.

This was an observational study of women with hypothyroidism referred to an endocrine antenatal clinic between 2008 and 2010 (n = 78; mean age, 30.4 years; range, 19 to 43 years). Thyroid profiles (free thyroxine [FT4] and TSH) before conception and through pregnancy were documented. Obstetrics outcomes were examined, including low birth weight, preterm births, preeclampsia, caesarean sections, and admissions to special care neonatal units.

Thyroid testing was undertaken in 80% of subjects before conception, and in 64, 94, and 96% of subjects in the first, second, and third trimesters of pregnancy, respectively. TSH >2.5 mU/L was seen in 49% of women before conception and in 68% of women in the first trimester. Six women were overtly hypothyroid before conception, attaining normal thyroid function at gestational ages ranging from 12 to 36 weeks. Neither the preconception nor the first postconception TSH level (>2.5 mU/L or ≤2.5 mU/L) was associated with gestational age at delivery, birth weight, or rates of caesarean section or preeclampsia.

The majority of women with hypothyroidism do not achieve the recommended preconception and early gestation TSH targets. Preconception and early gestation TSH >2.5 mU/L was not associated with adverse fetal and maternal outcomes. Studies in larger cohorts will be required to confirm these findings, however.